1. Field of the Invention
This invention relates generally to satellite broadcast systems and, more particularly, to a conditional access system for encrypting and decrypting data.
2. Related Art
A conditional access system is used to permit access to a transport stream only to subscribers who have paid for it. This is generally done by distributing the transport stream in encrypted form. Although any integrated receiver-decoder (IRD) that is connected to a satellite broadcast network can receive the encrypted transport stream, only the IRDs of those authorized subscribers are able to decrypt the encrypted transport stream. The IRD determines whether the encrypted transport stream should be decrypted and, if so, to decrypt it to produce a decrypted transport stream comprising information making up the broadcast program.
After a subscriber has purchased a service, a service provider sends messages to the subscriber's IRD with an authorization stream for the purchased services. The authorization stream may be sent with the transport stream or may be sent via a separate channel to an IRD. Various techniques have been used to encrypt the authorization stream. The authorization stream may include a seed as a key for a service of the service provider and an indication of what programs in the service the subscriber is entitled to receive. If the authorization stream indicates that the subscriber is entitled to receive the program of an encrypted transport stream, the IRD decrypts the encrypted transport stream using the received seed.
A well known problem concerning such conditional access systems is that the IRDs may suffer either carrier fades or be switched between carriers bearing the same instantiation of the service provider. It is therefore desirable for the IRDs to recover and pass a correctly decrypted transport stream to downstream processing stages as quickly as possible. However, the magnitude of time delay in the recoveries, on a typical large network (12,000 satellite IRDs) can be extremely long, such as one or two minutes in legacy systems. Other implementations of conditional access solve the problem of quick restoration of the IRD's decrypter by either risking that still-scrambled material may inadvertently be passed to the downstream processing stages, or consuming far more bandwidth in the transport stream to send cyphered seeds.
Hence, there is a need in the industry for an efficient and reliable technique for rapidly decrypting data after brief or extended loss of transport or authorization streams due to short carrier fades or switches. For that purpose, the conditional access system should allow the IRDs to quickly determine, after restoration of the data link following a carrier fade or switch, whether their stored copies of the decryption seeds are still current and correct. Furthermore, it is needed to greatly reduce the likelihood that the carrier fade or switch could prevent the IRD from getting at least one copy of its own messages without the need for consuming large amounts of bandwidth.